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Abstract:

A light emitting diode for harsh environments includes a substantially
transparent substrate, a semiconductor layer deposited on a bottom
surface of the substrate, several bonding pads, coupled to the
semiconductor layer, formed on the bottom surface of the substrate, and a
micro post, formed on each bonding pad, for electrically connecting the
light emitting diode to a printed circuit board. An underfill layer may
be provided between the bottom surface of the substrate and the top
surface of the printed circuit board, to reduce water infiltration under
the light emitting diode substrate. Additionally, a diffuser may be
mounted to a top surface of the light emitting diode substrate to diffuse
the light emitted through the top surface.

Claims:

1. A method for mounting a light emitting diode to a printed circuit
board in a flip chip orientation, comprising: removing a reflective layer
from a first surface of the light emitting diode; bonding a micro post to
each bonding pad on a second surface of the light emitting diode, the
second surface being opposite the first surface; bonding the micro posts
to corresponding bonding pads on the printed circuit board so that the
light emitting diode is mounted in a flip chip orientation; and mounting
a diffuser on the first surface of the light emitting diode to diffuse
the light emitted through the first surface of the light emitting diode.

[0003] A light emitting diode, or LED, is a semiconductor device that
emits a particular spectrum of incoherent light when a forward bias is
applied across the LED's anode and cathode terminals. An LED is formed by
doping a semiconductor material with various impurities to form a p-n
junction that emits photons when current flows from the p-side of the
junction (anode) to the n-side of the junction (cathode). The color, or
wavelength, of light emitted by an LED depends upon the material that
forms the diode's p-n junction. For example, an LED constructed from
aluminum gallium arsenide (AlGaAs) radiates infrared and red light, one
constructed from aluminum gallium phosphide (AlGaP) radiates green light,
one constructed from gallium nitride (GaN) radiates green and blue light,
one constructed from indium gallium nitride (InGaN) radiates near
ultraviolet, bluish-green and blue light, etc.

[0004] Generally, LEDs are formed by depositing a p-type layer onto an
n-type substrate. An anode pad, coupled to the p-type layer, is mounted
to the top surface of the LED chip, and a cathode pad, coupled to the
n-type substrate, is mounted to the top surface of the LED chip as well.
LEDs may also be formed on transparent substrates, such as sapphire
(Al2O3). For example, in a GaN-on-sapphire LED, at least one
n-type layer is first formed on an upper surface of the sapphire
substrate, and then one, or more, additional layers, including a p-type
layer, is formed on the upper surface to create the p-n junction. Many
well known processes may be used to form these layers, such as, for
example, metal organic chemical vapor deposition (MOCVD), plasma
deposition, etc. A reflective, metal layer is formed on the bottom
surface of the sapphire substrate to reflect downwardly-emitted light
back up through the top surface.

[0005]FIG. 1 depicts a prior art LED die 1 mounted to printed circuit
board 10, i.e., a "chip on board" design. A p-n junction 2 is formed on
the top surface of sapphire substrate 3, and a reflective, metal layer 4
is formed on the bottom surface of sapphire substrate 3. Wires 7, 8 are
bonded to the anode and cathode pads 5, 6 (respectively) on the top
surface of LED die 1, as well as to corresponding anode and cathode pads
11, 12 (respectively) on printed circuit board 10. Wire bonding, such as
thermocompression, thermosonic, ultrasonic, etc., is the standard method
by which wires 7, 8 are attached to LED pads 5, 11 and printed circuit
board pads 6, 12 (respectively). Metal layer 4 reflects
downwardly-emitted light from p-n junction 2 upward, ideally through the
top surface of LED die 1. Consequently, while some of the light emitted
by p-n junction 2 may escape through the sides of sapphire substrate 3,
most of the light is emitted from the top surface of LED die 1. FIG. 2
presents a picture of top view of LED die 1 mounted to printed circuit
board 10, showing wires 7, 8 bonded to LED pads 5, 6 (respectively).

[0006] LEDs have been used within in vivo, non-hermetically-sealed
sensors, and, in these applications, printed circuit board 10 is commonly
ceramic (alumina), or a ceramic composite, while LED substrate 3 is
typically sapphire, silicon or another similar material. While these
materials are generally impervious to water or water vapor, wires 7 and
8, LED pads 5 and 6 and printed circuit board pads 11 and 12 must be
protected from the harsh environment of the human body. Consequently,
these components are typically encased in a polymer material, which,
unfortunately is prone to water or water vapor infiltration. Over time,
this undesired water permeability not only affects the properties of the
polymer but also promotes premature failure of the LED by various
mechanisms, including, for example, dielectric constant degradation,
oxidation, electrical shorts, void space formation, delamination of gold
pads on substrates, etc.

[0007] FIG. 3 depicts the effect of water infiltration on a prior art LED
die, mounted to a printed circuit board, whose electric connections have
been encased in a polymer material. While printed circuit board 10
effectively blocks water from infiltrating into the electrical
connections to LED die 1 from the bottom, water may ingress into the
polymer material 15 from the remaining directions, as shown in FIG. 3. To
emphasize the water permeability problem, polymer material 15 has been
exaggerated in size in FIG. 3.

[0008] While it is known that LEDs can be mounted to printed circuit
boards in an inverted manner, i.e., a "flip chip" orientation, these
prior art techniques, by themselves, fail to overcome the problem of
water permeability when LEDs are deployed in a harsh environment.
Moreover, when compared to the standard, chip on board design, flip chip
LEDs emit less light because light that is emitted through the lower
surface of the LED, i.e., the surface that is closest to the printed
circuit board, is generally scattered, i.e., not reflected back into the
LED and out through the upper surface. Consequently, a prior art flip
chip LED not only fails to address the problem of water-permeability
within an in vivo, non-hermetically-sealed sensor, but also emits less
light than a standard, chip on board design.

SUMMARY OF THE INVENTION

[0009] In accordance with embodiments of the present invention,
semiconductor LED devices, chips, and chip-on-board circuitry in general,
are disclosed that are designed to withstand harsh environments,
including within non-hermetic circuitry packages, housings, and
encasements. In one embodiment, a semiconductor LED device includes a
substantially transparent substrate, a semiconductor layer deposited on a
bottom surface of the substrate, several bonding pads, coupled to the
semiconductor layer, formed on the bottom surface of the substrate, and a
micro post, formed on each bonding pad, for electrically connecting the
LED to a printed circuit board. An underfill layer may be provided
between the bottom surface of the substrate and the top surface of the
printed circuit board, to reduce water infiltration under the light
emitting diode substrate. In another embodiment, a diffuser may be
mounted to a top surface of the light emitting diode substrate to diffuse
the light emitted through the top surface.

[0010] In another embodiment, a semiconductor LED device is constructed
for use in an implantable sensor (such as a glucose sensor), wherein the
LED is contained within a non-hermetic encasement, and the implanted
sensor is operated for extended periods of time within a harsh
environment exposure. In one embodiment, an optical-based sensor for
determining the presence or concentration of an analyte in a medium
comprises an optically transmissive sensor body which functions as an
optic wave guide, wherein the sensor body has an outer surface
surrounding said sensor body. The sensor further includes a radiation
source in the sensor body which emits radiation within the sensor body,
wherein the radiation source includes a substantially transparent
substrate, a semiconductor layer deposited on a bottom surface of the
substrate, a plurality of bonding pads, coupled to the semiconductor
layer, formed on the bottom surface of the substrate, and a plurality of
micro posts, formed on the bonding pads, for electrically connecting the
light emitting diode to a printed circuit board. The sensor further
provides an indicator element having an optical characteristic that is
affected by the presence or concentration of an analyte, wherein the
indicator element is positioned on the sensor body to receive radiation
that travels from the radiation source, and which transmits radiation
into the sensor body. The sensor also includes a photosensitive element
located in the sensor body and positioned to receive radiation within the
sensor body and which emits a signal responsive to radiation received
from said indicator element.

[0011] In another embodiment, a method for mounting a light emitting diode
to a printed circuit board in a flip chip orientation is disclosed which
comprises (1) removing a reflective layer from a first surface of the
light emitting diode; (2) bonding a micro post to each bonding pad on a
second surface of the light emitting diode, the second surface being
opposite the first surface; (3) bonding the micro posts to corresponding
bonding pads on the printed circuit board so that the light emitting
diode is mounted in a flip chip orientation; and (4) mounting a diffuser
on the first surface of the light emitting diode to diffuse the light
emitted through the first surface of the light emitting diode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The above and other advantages of this invention will become more
apparent by the following description of invention and the accompanying
drawings.

[0013]FIG. 1 depicts a prior art LED mounted to a printed circuit board.

[0014] FIG. 2 presents a picture of a top view of the prior art LED
mounted to a printed circuit board.

[0015] FIG. 3 depicts the effect of water infiltration on a prior art LED
die, mounted to a printed circuit board, whose components have been
coated with a polymer material.

[0016] FIG. 4 depicts the limited effect of water infiltration on a flip
chip LED die mounted to a printed circuit board, in accordance with an
embodiment of the present invention.

[0017] FIGS. 5A and 5B depict an unmasked, prior art printed circuit board
and a printed circuit board masked in accordance with an embodiment of
the present invention, respectively.

[0018] FIG. 6 depicts a process for mounting an LED die in a flip chip
orientation on a printed circuit board using thermosonic bonding, in
accordance with an embodiment of the present invention.

[0019] FIG. 7 presents an electron micrograph depicting an underfill
containing a mixture of glass microballoons and cyanate ester, in
accordance with an embodiment of the present invention.

[0020] FIG. 8 depicts a flip chip LED die thermosonically bonded to a
printed circuit board with an underfill layer, in accordance with an
embodiment of the present invention.

[0021] FIG. 9 presents a schematic of a diffuser mounted to a flip chip
LED, in accordance with an embodiment of the present invention.

[0022] FIGS. 10A and 10B depicts a picture of a flip chip LED with a
diffuser, in non-energized and energized states, in accordance with an
embodiment of the present invention.

[0023] FIG. 11 presents a flip chip LED mounted within a sensor, in
accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

[0024] Embodiments of the present invention provide semiconductor LED
devices, chips and chip-on-board circuitry that are designed to withstand
harsh environments within non-hermetic, or near-hermetic, circuitry
packages, housings, and encasements, as well as a diffuser that
advantageously spreads out the light emitted by the LED in order to
excite a maximum number of sensor indicator molecules.

[0025] A harsh environment, as contemplated by the present invention,
includes various moisture-laden, or hygroscopic, environments, such as,
for example, submerged applications, such as underwater or undersea
sensors, implanted medical applications, such as glucose sensors, outdoor
applications that are subjected to extremes of humidity and rain, etc.
The LED device may be subjected to continuous, as well as intermittent,
exposure to water or water vapor within these harsh environments.

[0026] Preferred embodiments of the present invention provide a flip chip
LED die that is bonded to a printed circuit board substrate, an underfill
layer between the flip chip LED die and the printed circuit board
substrate to prevent water infiltration and enhance light reflectivity.
In still other preferred embodiments, the present invention provides a
flip chip LED that utilizes a diffuser to enhance light distribution. The
flip chip orientation advantageously increases the far field radiation
pattern of the LED and positions the bulk of the LED die above the bonds
between the LED die and the printed circuit board, which protects these
electrical connections, to a large degree, from the harsh environment. As
noted above, the preferred embodiment may be employed within an
implantable sensor which detects various analytes of interest within the
body, such as glucose. In this embodiment, the flip chip LED is contained
within a non-hermetic, or near-hermetic, encasement and operated for
extended periods of time within a harsh environment, i.e., the human
body. The flip chip is bonded to the printed circuit board substrate
using any one of a number of techniques, such as thermosonic bonding,
thermocompression bonding, ultrasonic bonding, welding, etc. Thermosonic
bonding, for example, offers an important advantage for medical use over
solder, as described in the prior art, because gold is welded to gold
using ultrasonic energy. Gold, of course, is biologically inert.

[0027] FIG. 4 illustrates a flip chip LED die 30 mounted to printed
circuit board 20, in accordance with an embodiment of the present
invention. Flip chip LED die 30 includes a p-n junction 32 formed on a
sapphire substrate 33, gold anode and cathode pads 5, 6 coupled to p-n
junction 32, gold micro posts 27, 28 thermosonically-bonded to LED pads
5, 6 and gold printed circuit board pads 21, 22 (respectively). FIG. 4
also depicts the limited effect of water infiltration on flip chip LED
die 30 mounted to printed circuit board 20. Not only does printed circuit
board 20 block water from permeating the thermosonically-bonded
connections from the bottom, but LED substrate 33 itself blocks water
from permeating to these connections.

[0028] Because water permeability is dependent upon the surface area of
the diffusible material, only a narrow, exposed diffusible surface area
16 is presented to the harsh environment, as depicted in FIG. 4. In one
embodiment, the height of the gap between LED substrate 33 and printed
circuit board 20 is preferably about 20-60 μm, more preferably about
30-50 μm, and most preferably about 40 μm. In one embodiment, the
diffusible surface area 16 of flip chip LED 30 is preferably about 0.01
mm2 to about 1 mm2, more preferably about 0.02 mm2 to
about 0.09 mm2, and most preferably about 0.05 mm2. The product
of the gap height and the die perimeter yields the diffusible surface
area, which is significantly less (e.g., by a factor of 103) than
the surface area of prior art LED die 1 presented within FIG. 3.

[0029] Generally, printed circuit board 20 is designed to minimally expose
metal conductor traces, and, preferably, to expose only the contact area
directly beneath the components mounted on the substrate, i.e., the flip
chip LED, supporting chips, passive circuit elements, etc. In one
embodiment, only critical metallization is exposed, such as, for example,
high impedance traces, pads, circuit routing, etc. This is accomplished
by careful layout and masking of all metal conductors to expose only
metal that is absolutely required to electrically connect the essential
components. Advantageously, the exposed metal will be substantially
covered by the components mounted on the surface of the printed circuit
board substrate. FIG. 5A depicts an unmasked, prior art printed circuit
board 10, having pads 17. FIG. 5B depicts a printed circuit board 20,
having pads 23 which is masked in accordance with an embodiment of the
present invention.

[0030] FIG. 6 depicts a process for mounting a commercial LED die in a
flip chip orientation on a printed circuit board using thermosonic
bonding, in accordance with an embodiment of the present invention. In
one embodiment, commercial LED die 1 includes p-n junction 2, sapphire
substrate 3, metal layer 4 and pads 5, 6, such as, for example, a Nichia
377 nm LED die, a Fox 360 nm LED die, etc. In a preferred embodiment, the
die size of the LED is about 310 μm to 320 μm, while in other
embodiments, the die size may be larger or smaller. Pads 5, 6 are
preferably separated by about 5 mm. In step 1, the bottom metal layer 4
is removed from LED substrate 3 using an appropriate method known to
persons skilled in the art, such as, for example, acid etching, including
a mixture of sulfuric acid and hydrogen peroxide (i.e., a piranha bath),
base etching, etc. In step 2, gold micro posts 27, 28 are welded, e.g.,
thermosonically bonded, to pads 5, 6 (respectively). View A presents a
picture of micro posts 27, 28 after bonding to pads 5, 6. In step 3, LED
die 1 is inverted to become flip chip LED die 30, and gold micro posts
27, 28 are thermosonically bonded to gold pads 21, 22 on printed circuit
board 20.

[0031] In a prior art flip chip application, an underfill material may be
provided between the flip chip and the printed circuit board. The purpose
of this underfill material is to compensate for thermal expansion
mismatch between the flip chip and the printed circuit board, and to
reinforce the adhesion of the soldered, flip chip to the printed circuit
board. Similar to the prior art polymer coating material discussed above,
water or water vapor permeates into this underfill material and alters
the underfill material's dielectric constant, which can cause premature
failure of the prior art flip chip. Typical commercial underfill
materials permit substantial water vapor diffusion, and are,
consequently, inapplicable for use in a harsh environment.

[0032] Embodiments of the present invention provide an underfill material
that is substantially impermeable to water vapor diffusion and,
therefore, are appropriate for use in a harsh environment. In other
words, the present invention provides an underfill material that has an
extremely reduced and minimal permeability. Generally, the underfill
material is a polymer, such as, for example cyanate ester, Epo-Tek 301-2,
etc., to which a filler, such as, for example, solid or gas-filled glass
microballoons, glass or latex microspheres, white alumina ceramic,
titanium(IV) oxide, etc., is added. The weight percentage of filler to
the total weight of the composition is, generally, from about 5% to about
70% w/w, and the corresponding weight percentage of polymer to the total
weight of the composition is from about 95% to about 30% w/w. In a
preferred embodiment, the filler is about 30% w/w and the polymer is
about 70% w/w.

[0033] In an embodiment, the underfill material is a syntactic foam,
formed by adding glass microballoons to a polymer with a very low
dielectric constant and very low intrinsic water vapor diffusion
properties, such as cyanate ester. In this embodiment, the solid glass or
ceramic microballoons work well to thwart vapor infusion and provide
dielectric optimization, as discussed below. The microballoons may have
any suitable size. For example, the microballoons may be as small as
about 2 μm to as large as about 11 μm in cross section. In one
embodiment, an adhesion promoter may be used to improve adhesion between
the organic and inorganic components of the syntactic foam.

[0034] In a preferred embodiment, silane is used to promote adhesion
between the glass microballoons and the cyanate ester. In this
embodiment, a solution of 95% ethanol and 5% water is adjusted to a pH
level of about 5, and approximately 2% volume-to-volume (v/v) of
Glycidoxypropyltrimethoxysilane (GPS) is added. The microballoons are
added to a beaker containing this solution and agitated (e.g., stirred),
separated from the solution by vacuum filtration, dried for about 1 hour
at about 110° C., and then added to the cyanate ester along with a
small, additional amount of GPS, and, optionally, a small amount of
colorant. Other silane-based adhesion promoters, non-silane-based
adhesion promoters, bridging reagents, etc., may also be used.

[0035] Generally, a dielectric is defined as an insulating medium between
two conductors, i.e., a material that is an electrical insulator. The
dielectric constant for a material is defined as the ratio of the
permittivity of the material to the permittivity of free space, and a
good insulator has a dielectric constant below 5. For example, the
dielectric constant for air is 1, solid polypropylene is 2.18, FR-4 (a
common circuit board composite) is 4.5, water is 80, cyanate ester is
2.7, ice is 3.2, paraffin is 2, and glass is 3.8. The dielectric constant
of a mixture of materials is simply a weighted average of the dielectric
constants of the components. For example, from the values listed above,
the dielectric constant of snow, comprised of ice, water and air, may be
represented as follows: (% air)*(1)+(% water)*(80)+(% ice)*(3.2).
Although air and ice both have good insulator values below 5, it is
obvious that the relative percentage of water is most influential on the
final insulative properties of snow.

[0036] In a preferred, high-impedance embodiment, the underfill layer is
made from cyanate ester, having a dielectric constant of 2.7, glass,
having a dielectric constant of 3.8, and air, having a dielectric
constant of 1, which are all below 5. Titanium oxide may include in the
underfill layer for maximum optical reflectivity where high impedance
protection may not be required. As water vapor enters the underfill layer
under normal diffusion, it will affect the dielectric constant of the
underfill layer. In one embodiment, each glass microballoon is an air
filled sphere having a diameter of about 11 μm and a wall thickness of
about 1 μm. When cyanate ester is filled with microballoons (e.g.,
about 30%), these glass spheres reduce the volume which is permeable to
water vapor, which, of course, would be same if the glass spheres were
solid. The air within the microballoons remains dry and stable with a
very low dielectric value (i.e., 1). Thus, the fraction of the underfill
layer which is permeable to water vapor is greatly reduced, i.e., only
the cyanate ester material and not the microballoons. The dielectric
constant of the underfill layer may be represented as follows: (%
air)*(1)+(% cyanate ester)*(2.7)+(% glass)*(3.8).

[0037] In this embodiment, only the cyanate ester fraction is susceptible
to variability due to water vapor encroachment. The glass, as well as the
air sealed within the glass, is unaffected by the water vapor. As noted
above, cyanate ester also has a very low water vapor permeation
coefficient. As water encroaches, the effective value of cyanate ester
increases somewhat; however, the increase is greatly offset by the high,
dry, and stable air component, and the composite is maintained below 5 at
maximum saturation of water vapor, and thus maintains good insulative
properties.

[0038] FIG. 7 presents an electron micrograph depicting an underfill
containing a mixture of glass microballoons and cyanate ester, in
accordance with an embodiment of the present invention. In this
embodiment, the glass microballoons are air-filled, have a diameter of
about 11 μm and a wall thickness of about 1 μm, and have been
pre-treated with silane, as described above, to improve adhesion with the
cyanate ester. The weight percentages for this embodiment are about 30%
w/w microballoons, 67% w/w cyanate ester, 2% w/w GPS and 1% w/w black
colorant.

[0039] In another embodiment, the underfill layer is formed by adding a
light-scattering filler, such as titanium(IV) oxide, glass microspheres,
etc., to an optical epoxy, such as Epo-Tek 301-2. In this embodiment, the
light-scattering filler advantageously increases the reflectivity of the
underfill layer. An adhesion promoter may also be used as well.

[0040] FIG. 8 depicts a flip chip LED die thermosonically bonded to a
printed circuit board with an underfill layer, in accordance with an
embodiment of the present invention. Underfill layer 40 substantially
prevents water or water vapor from permeating to LED micro posts 27, 28
and pads 5, 6, and printed circuit board pads 21, 22. Underfill layer 40
may be combined with a flip chip mounting orientation to prevent water
incursion for other circuitry components mounted on printed circuit board
20, such as microelectronic dies, common integrated circuits, ASICs, etc.
The various components of the glucose sensor described herein may be
mounted in just such a fashion.

[0041] FIG. 9 presents a schematic of a diffuser mounted to a flip chip
LED, in accordance with an embodiment of the present invention. Flip chip
LED die 30 is thermosonically bonded to printed circuit board 20 and has
an underfill layer 40 as well as a diffuser 50 mounted to the upper
surface. In one embodiment, diffuser 50 is shaped in the form of a dome
or hemisphere to distribute the light emitted through the upper surface
of flip chip LED die 30 over a wider area, and is, generally, made from a
polymer, such as, for example, Epo-Tek 301-2. In other embodiments, the
diffuser 50 could have other shapes as well that improve light
distribution. Additionally, a light-scattering filler, such as glass
microballoons, titanium(IV) oxide, etc., may be incorporated into
diffuser 50 to improve light distribution, similar to the underfill layer
discussed above. FIGS. 10A and 10B depict pictures of a flip chip LED
with diffuser in non-energized (FIG. 10A) and energized states (FIG.
10B). The diffuser of the present invention substantially increases the
far-field emission pattern of the light from the LED die and results in a
more symmetrical distribution of light. The diffuser may be formed
directly on flip chip LED 30 by transferring a predetermined amount of
the polymer/filler mixture to the upper surface of flip chip LED 30 using
a pipette, etc., or, alternatively, a pre-formed diffuser may be attached
to the upper surface of flip chip LED 30.

[0042] FIG. 11 presents a flip chip LED mounted within a sensor, in
accordance with an embodiment of the present invention. In this
embodiment, sensor 60 is a sensor to monitor in vivo analytes of interest
within a patient, such as for example, glucose levels. Flip chip LED die
30 is mounted to the top surface of printed circuit board substrate 62,
in accordance with the techniques described above. Photodetector 64 and
supporting electronics 66 are also mounted on printed circuit board 20,
preferably in accordance with the techniques described above. The printed
circuit board 62 is enclosed by a non-hermetically sealed housing 68, to
which fluorescent indicator molecules 70 are bound on some or all of the
outside surface of the housing. Flip chip LED die 30 illuminates
indicator molecules 70 at a particular wavelength, which fluoresce, at a
different wavelength, based upon the concentration of a particular
analyte in the body of the patient (e.g., glucose). The fluorescent light
emitted by fluorescent indicator molecules 70 is measured by
photodetector 64, and a signal is then detected by a receiver located
outside the patient's body. Many different sensor architectures will
benefit from the teachings of the present invention, including, for
example, U.S. Pat. No. 6,330,464 (Colvin et al.), the disclosure of which
are incorporated herein by reference in its entirety. In this embodiment,
the LED die with the diffuser is configured to have a more symmetrical
distribution of light which excites more indicator molecules on the
surface of the sensor.

[0043] In various embodiments, the present invention allows substrate
printed circuitry, integrated chip circuitry, and specifically, an LED
die to withstand harsh environment and submerged application using
non-hermetic or near hermetic polymer encasements. The present invention
also enables use of non-hermetic circuitry encasements which are
typically less expensive to manufacture, can be miniaturized to a greater
degree, and can be more suited to medical implant applications. Various
embodiments of the present invention further eliminate flip-chip
installation use of solder and flux as described in prior art flip-chip
applications. For example, the embodiments of the present invention use
gold welded microposts instead of solder and flux, which is important for
both medical use so as to not leach solder components such as lead. This
is also beneficial for improving electronic reliability in harsh
environments because of flux residue and the difficulty, or
impossibility, of adequately cleaning flux residue from the substrate and
beneath chips as exists with solder bumping.

[0044] Various embodiments of the present also provide intrinsic
protection of board level metallization using the chip body itself as a
moisture diffusion barrier and minimizes diffusible area of exposed
polymer composite underfills to 20-70 microns×chip circumference.
This results in 100's and 1000's fold reduction in polymer area exposed
and vulnerable to water vapor diffusion from outside the device.

[0045] In further embodiments, pretreatment of any flip chip die and/or of
the circuit board with an adhesion promoter prior to underfill increases
the bond strength of the inorganic/organic interface, particularly under
harsh environments, when water may otherwise cause delamination of the
underfill.

[0046] In still other embodiments, the diffuser constructed onto the LED
die allows better distribution of excitation light within an implantable
sensor construct to excite more indicator molecules and provide a much
higher signal to noise ratio for the sensor.

[0047] Other benefits of the present invention include improved electronic
reliability. In one aspect embodiment of the invention, this is
accomplished by the use of gold metallization and thermosonic welding,
which provides better biocompatibility, is not prone to metal oxidation
over time, and there are no toxic metals as with solder.

[0048] In further embodiments of the invention, the underfill composite
material for flip-chipped LED die is designed to provide maximum
reflectance and optimize light yield and minimize water vapor
encroachment. The underfill composite material is also designed for
application in harsh environments to minimize water vapor encroachment
and maintain optimal dielectric value.

[0049] In still further aspects of the invention, methods for designing
circuit or microcircuit "chip-on-board" substrates to be used in harsh
environments, as well as processes for assembling circuits intended for
harsh environments, are disclosed. Also, low pass filter to filter "red
tail" emissions from LED and higher quality bandwidth emission are
provided.

[0050] While this invention has been described in conjunction with
specific embodiments thereof, many alternatives, modifications and
variations will be apparent to those skilled in the art. Accordingly, the
preferred embodiments of the invention as set forth herein, are intended
to be illustrative, not limiting. Various changes may be made without
departing from the true spirit and full scope of the invention as set
forth herein.